Embodiments of the invention relate to processes for improving the recovery of precious metals specifically gold and silver from recalcitrant refractory ores by use of hydrofluoric acid dissolved in ionic liquids.
WO 84/04759 discloses a method to extract metals from ores which exist in the form of oxides or sulphides, by treating the ore with hydrogen fluoride and/or fluorosilicic acid. When hydrogen fluoride is used, the ore is initially crushed, dried and then contacted with hydrogen fluoride gas at temperatures above 105 C to produce metal fluorides. The metal fluorides are then contacted with aqueous hydrogen fluoride solution and the resulting insoluble metal fluorides and iron oxides are separated from the solution, and metal fluorides preferably including nickel and cobalt fluorides are recovered.
U.S. Pat. No. 4,923,507 discloses a process for opening ores, particularly ores containing tantalum, niobium, zirconium and titanium, wherein the ore normally undergoes stages of leaching with mineral acids, solvent extraction, purification and separation to the metals. Instead of directly leaching with sulfuric acid and hydrofluoric acid as a first stage, the reference describes generating the hydrogen fluoride in-situ by melting the ore together with fluorite (CaF2), followed by milling of the melting product obtained and subsequently leaching with concentrated sulfuric acid only.
More recently Lawrance et al have described in the journal Green Earth. Vol 6, page 313-315, 2004 a process to use ionic liquid to recover gold and silver by making use of iron sulfate as an oxidant and thiourea as a leaching agent. The gold and silver extracted is recovered from the ionic liquid using activated charcoal.